Accelerated Corrosion of Paints - Q-Lab

REPRINT
May 1991
Accelerated Corrosion
Testing of
Industrial Maintenance
Paints Using A
Cyclic Corrosion
Weathering Method
by C.H. Simpson, C.J. Ray, and B.S. Skerry
The Sherwin- Williams Co.

Accelerated
Corrosion Testing
of Industrial
Maintenance
Paints Using A
Cyclic Corrosion
Weathering Method
by
C.H. Simpson,
C.J. Ray, and
B.S. Skerry
The Sheuwin-
Williams Co.
I t is widely accepted that improved
methods for assessing the corrosion
controlling properties
of organic coatings on steel are
needed. Difficulties ~- -. with established testing
procedures (primarily ASTM B 117 salt spgy) have
been well documented]-3 and need not be
restated here. As pointed out in a recent review,4
the need for reliable short-term testing procedures
is greater than ever considering the extensive
formulation changes in established coating systems
that are expected to result from increasingly
stringent volatile organic content (VOC)
regulations. More realistic testing procedures
will allow meaningful performance comparisons
between old and new VOC-compliant compositions
without reliance on long-term exterior
exposure results.
The most crucial requirements of a "good"
or meaningful laboratory test are that it simulates
the relative performance rankings of materials observed
in practice and that it produces failure
modes consistent with field experience. Additionally,
a useful test must be reproducible and reasonably
rapid. No currently available tests have been
shown to meet these requirements.5
Although it would not seem practical (or
even possible) to simulate, completely, the numerous
and complex variables operating in the outdoor
environment, a reasonable approach would
Journal of Protective Coatings 6 Linings

Accelerated Testing of Maintenance Coatings
(b) alkydalkyd
(c) latefiatex
Fig. 1 Scribed regions ofpanek atler2,OOO hours'$alt spmy testing (1,000 hours for lalex)
(a) epoxylepoxy
(b) alkydalkyd
Fig. 2 Scribed regions of panels after cyclic wetldry corrosion fesling (2,000 hours)
involve identifying the most significant of these
variables and incorporating them into an accelerated
testing protocol. In recent years, claims of improved
correlations with exterior results have been
reported based on the use of cyclic wetldry corrosion
chambers6Jl8 (i.e, chambers producing periods
of salt fog alternating with periods when test
samples are allowed to dry). This cyclic wetting
and drying of electrolyte layers from the panel surface
is thought to stress the coating in a more realistic
manner than, for example, a continuous
ASTM B 117 salt spray test, where panels are
placed in a constant, high relative humidity (RH)
environment (-97 percent). At the very least, incorporation
of wetldry cycling factors seems intuitively
justified, considering that materials exposed
to the outdoor environment undergo similar wetting
and drying effects on a frequent basis. The importance
of electrolyte composition has also been
addressed (e.g., the incorporation of ammonium
and sulphate species to simulate corrosion processes
occurring in industrial environmentsl~6).
Although there has been increased recognition
of the importance of wetidry cycling (or cyclic
stress factors in generalg) and electrolyte composition
variables, little attention has been given to the
influence of weathering factors (e.g., ultraviolet
light exposure, moisture condensation) in the
overall paint degradation and corrosion process.
Because corrosion at the paint/metal interface and
weathering of paint films are processes occurring
simultaneously in nature, it does not seem unreasonable
to suggest that the 2 processes may be significantly
interrelated. For example, weather-induced
degradation of a paint's organic binder may
result in a more hydrophilic coating surface, which
could change the time-of-wetness and subsequent
corrosion characteristics of the system.
Other effects may include the following:
enhanced retention of surface contamination
and transport of detrimental species through the
weather-damaged coating;
changes in physical properties of the paint, such
as elasticity, which may have an impact on subsequent
performance characteristics; and
possible dilution of surface species and other effects
associated with the deposition of relatively
"clean" water on the painted surface during periods
of rainfall or condensation.
In an earlier paperlo, the significance of
combining weathering and corrosion cycles into a
single test method was investigated. The results indicated
that the corrosion performance characteristics
of organic paint films were markedly affected

Accelerated Testing of Maintenance Coatings
(a) epoxylepoxy (b) alkydlalkyd (c) latexllatex
Fig. 3 Scribed regions of panels aller combined corrosionlweathering testing (2,000 hours)
(b) alkydlalkyd
Fig. 4 Scribedregions orpanels affer 27months1marine exposure (21 months for latex)
by the ultraviolet light-weathering factors in the
test. The purpose of this paper has been to compare
results obtained from a combined cyclic corrosiodweathering
test method with data obtained
from outdoor exposures. Further comparisons with
data from ASTM B 117 salt spray and wetidry corrosion
cycling (in the absence of the weathering
factors) have also been made.
Experimental Design
Coating Systems
Three commercial quality, industrial maintenance
coating systems (i.e., primers with appropriate topcoats),
representing important generic coating
types (catalyzed epoxy-polyamide, acrylic latex, and
alkyd), were studied. Further details for these coating
systems are shown in Table 1.
All coatings were applied to grit-blasted,
3 in. by 6 in. (8 cm by 16 cm) cold rolled steel
test panels (washed and degreased) using an
automated air spray technique. Primers were
allowed to cure 24 hours before topcoating.
Topcoated panels were allowed to cure for
1 full week before testing. The lower half of each
coated panel was scribed with an X through
the coating down to the metal substrate. (Panels
exposed at the marine site were scribed with a
single straight line, running vertically along the
panels.) In all cases, panels were exposed in
duplicate pairs.
Accelerated Testing Procedures
Three accelerated tests were studied:
standard ASTM B 117 salt spray,
cyclic wettdry corrosion testing using a dilute
(NH4)2S04/NaC1-based electrolyte, and
the cyclic wetldry corrosion test (as above) with
weathering factors incorporated (i.e., a combined
corrosiodweathering test).
Table 2 outlines the important features of
these testing procedures. A more detailed description
of each technique is given below. Coating systems
were tested for 2,000 hours in each accelerated
exposure environment.
Salt Sprag (ASTM B 11 7-85)
Panels were exposed, at a 15- to 30-degree angle
Journal of Protective Coatings & Linings
3

Accelerated Testing of Maintenance Coatings
Table 1 Coating Systems Studied
Total DFT
Primer Topcoat Substrate mils (microns)
(barium metaborate
inhibitor)
(cold-rolled, 1.5- to 2.0-mil
Table 2 Salient Features of Test Methods Studied
Test 'b~e Features
Wetidry test** Cyclic stress I-hour 0.35 (wt) percent (NH4)2S04,0.05 (wt)
percent NaCl spray at ambient temperature;
I-hour dry at 35 C, purged with air;
*PBTASTM B 117
**WeUdrpFmhesion~ Mebon Paints, UUd)
w*Com'~eat~W cabinet (@Panel Cornpang)
from the vertical, to continuous deposition of a ing with one-hour periods with no spray and eleneutral
5 (wt) percent NaCl solution at elevated vated temperature (35 C). The electrolyte used in
temperature (35 C) and high humidity (approxi- this work was a 0.35 (wt) percent (NH4)2S04, 0.05
mately 97 percent RH).
(wt) percent NaCl solution having a slightly acidic
pH (-5.2) upon atomization. Electrolyte was
WetIDuy (Mixed Salt) Corrosion Test
atomized at an approximate rate of 600 mhour
Panels were exposed to one-hour periods of salt into a 0.33 m3 testing chamber. Panels were exspray
(mixed salt) at ambient temperature alternat- posed at a 15- to 30-degree angle from the vertical

Accelerated Testing of Maintenance Coatings
Performance Ratings* of
lhble 3 Coating Systems After Testing
Test Method Conditions Rated Alkyd System Latex System Epoxy System
Cyclic wetldry Blistering (size, frequency) 10110 10110 10110
corrosion test Rust-through 10 10 10
(2,000 hours) Undercutting 7 7 7
Overall 37 37 37
marine site Rust-through 10 10 10
(27 months) Undercutting 5 7 0
Overall 35 37 30
Ratingsyslem described in results section
on shelves attached to the inside walls of the testing
chamber. By this arrangement, the panels were
positioned in close proximity to resistive heaters
that were located in the chamber walls. The resistive
heaters were activated during drying periods,
thus evaporating the electrolyte layers from the
panel. surfaces. During these dry periods, the cabinet
was also purged with air at a regulated flow
rate. This cyclic wet/dry testing procedure is essentially
that developed by Timmons in the 1970s.6
Chambers designed for carrying out this test are
commercially available.
Cyclic Corrosion/
Weathering Test
Panels were exposed to 200-hour periods of wet/dry
corrosion cycling (as described above) followed by
200-hour periods of ultraviolet light condensation
exposure for a total of 2,000 hours (i.e., 5 complete
wet/dry/ultraviolet light condensation cycles). For
the weathering component of this test, a standard
ultraviolet light condensation cabinet, conforming
to the ASTM G 53 standard, was employed. Here, a
four-hour ultraviolet light exposure period, using
WA-340 bulbs, at 60 C, was cycled with a fourhour
condensation period at 50 C.
Outdoor Exposure Sites
Marine Site
Panels were exposed for up to 27 months at a 45-
degree angle from the vertical, facing east at a testing
site located on the eastern shore of Florida
(Ponce Inlet).
Industrial Site
Panels were exposed for 12 months at a 45-degree
angle from the vertical, facing south, on the rooftop
of our company's technical center in downtown
Cleveland, OH. The conditions at this site are corrosive,
based on weight-loss measurements made
on bare steel samples (e.g., removal rates [short
term] greater than 50 micrometerslyear for mild
steel are typical).
Journal of Protective Coatings 6 Linings

Accelerated Testing of Maintenance Coatings
Results
The rankings predicted by the combined
corrosion/weathering tests were found to be most
Almost without exception, all failures observed
consistent with those observed in the field. In conafter
testing (accelerated and natural) were associ- trast, the salt spray test predicted precisely the opated
with corrosion and delamination effects along
posite ranking observed in practice. No clear differthe
panel scribes. Figs. 1-4 illustrate the appearentiation
of performance was possible using data
ance of the scribed regions of the coated panels
from the wetldry corrosion test.
after salt spray, cyclic wetldry corrosion, combined
corrosion/weathering, and marine site exposure
tests, respectively. The epoxylepoxy system was the
only material showing any signs of degradation
after 12 monthsJ exposure in an industrial atmo- Rankings predicted
sphere. This degradation is illustrated in Fig. 5.
Table 3 shows the average ASTM blister (D
714), rust-through (D 610)~ and undercutting (D
by the combined
1654) ratings assigned to the coating systems after
2,000 hours of accelerated testing and after field COITOS~O~/W~~ thering
testing at the marine and industrial sites. In these
rating systems, "10" indicates perfect performance, tests were
and "0" indicates total failure.
Also reported in Table 3 is an overall perfor- most consistent
mance index, calculated by summing the individual
ratings for blister size, blister frequency, under- with rankings
cutting, and rust-through. (The descriptive ratings
suggested by ASTM for blister frequency were conin
the field.
verted to numeric ratings as follows: none=lO,
few=7.5, medium=5, medium-dense=2.5, and
dense=O.) The best possible overall rating - was
therefore 40.
The overall performance values were
Importantly, these rankings, derived from
the data in Table 3, were consistent with the overthen
used t' rank the performance of the
coating systems (on a test-by-test basis), as shown
all visual appearance of the coatings after testing,
in Table 4.
as illustrated in Figs.
Discussion
Rank Correlations
From Table 4, several important points may be
observed.
Each laboratory test method produced a
unique ranking of materials. This suggests that the
differences between these test methods (i.e, continuous
NaCl salt spray, wetldry (NH4)2S04/NaCI salt
spray, and wetldry (NH4)2S04/NaC1 salt spray with
incorporation of weathering factors) cause quite
distinct variations in the corrosion protection and
degradation characteristics of these organic coatings.
The rankings observed at the marine and industrial
field sites were similar, in that the
epoxylepoxy panels exhibited the most severe
degradation at both sites.
Modes of Degradation
All samples exposed at the marine site were found
to undergo blister formation in areas adjacent to
the panel scribes. These blisters were stable (i.e.,
did not collapse upon storage or upon application
of finger pressure), were rust-filled, and were
stained orange-brown by the run-off from adjacent
corrosion products. In some cases, the scribe line
underwent a general lifting due to the accumulation
of solid corrosion products visible within.
These results are illustrated in Figs. 4(a)-(c). A
similar type of scribe line lifting and delamination
associated with nearby blisters was also observed
on the epoxylepoxy system after 1 year at the industrial
atmospheric test site, as illustrated in Fig.
5. Although a very slight tendency to produce filiform
corrosion was observed on the latextlatex
sample after marine site exposure (Fig. 4(c)), this
was not generally found to be a significant mode of
failure in the natural exposure environments.
The modes of degradation observed at the

Accelerated Testing of Maintenance Coatings
Wle 4 Rank Correlations
Table 5
Exposure Condition
WeWdry corrosion test
Exterior (marine)
terior (ifidustrial)
-
Ranking Observed
(best to worst)
Alkyd=latex=epoxy
Latex>alkyd>epoxy
Latex=alkydrepox
Performance Ratings* of
Latex Coatings After
1,200 Hours
Cowosion/Weathering Test
Blistering
Coating
(size1
frequency)
Rust-
Through Undercutting
B 10110 6 8
D loll0 4.5 7
*Rating system descdbed in resulls section
marine field sites were generally quite similar to
those observed after testing using the combined
corrosion/weathering test (Fig. 3). This was apparent,
particularly when comparing the marine site
exposures with equivalent panels after 2,000 hours
of corrosionhveathering testing. For example, with
the epoxylepoxy system, both field and laboratory
samples exhibit relatively large (greater than 1 cm
in diameter), low-profiled, rust-filled blisters
'
("scabs") along the scribes. (Compare Fig. 3(a)
with 4(a).) Further, a comparison of the performance
of the latexflatex system after marine exposure
and after corrosion/weathering testing shows
that both samples exhibit only very small rustfilled
blisters (less than or equal to 2 mm in diameter),
negligible loss of adhesion, and only slight
rust-staining immediately adjacent to the scribe.
(Compare Fig. 3(c) with 4(c).)
Neither the salt spray test nor the cyclic
wet/dry corrosion test (Figs. 1 and 2, respectively)
was particularly successful in reproducing the
types of failures observed after field testing. The
wet/dry corrosion test produced some lifting of the
coating along the scribe and had a clear tendency
to produce filiform corrosion (e.g. Fig. 2(c)). The
formation of filiform-type corrosion failures in this
cyclic wet/dry corrosion environment is consistent
with the findings of other workers798 Exposure in
the salt spray chamber generally resulted in an accumulation
of loosely adherent corrosion products
covering the scribe and, for the latedatex system,
produced a particularly severe (and unrealistic)
breakdown of the coating through the formation of
blisters over the entire panel within 1,000 hours of
testing. These blisters were of a different nature
than those formed along the scribes during exterior
exposure and during corrosion/weathering tests.
In the salt spray test, the blisters tended to be
more hemispherical (i.e., had greater profile) and
were not filled with solid corrosion products, as
was observed after field testing, but were often
filled with liquid electrolyte. The formation of rustfilled
blisters around scribed regions of the coatings
was not significant in either the salt spray or
the wetldry cycle test.
Based on the relatively good performance of
the latex system after field exposure, Besalt spray
test would appear particularly misleading in the
evaluation of these water-borne systems. As a point
of interest, the poor salt spray resistance of latex
coatings may, to some extent, account for the general
reluctance to specify such water-borne paints
for service in corrosive atmospheres. The high performance
capabilities of these water-borne systems,
however, are increasingly being recognized311
This further demonstrates the need for realistic
and meaningful laboratory testing procedures.
Evaluation of Closely Formulated Coatings
Clearly, from the results above, the most meaningful
assessments of performance were obtained from
the test incorporating both wet/dry cycle corrosion
and ultraviolet light condensation weathering fac-
Journal of Protective Coatings 6 Linings
7 '

Accelerated Testing of Maintenance Coatings
L
-. 5 Scribed region of epoxylepo -
-
lstem afler 12 months' ex1
1
exposure (industrial)
la) initial latex formulation (b) latex with added corrosion inhibitor (c) latex with different surfactant added
Fig. 6 Acrylic latex-ma fedpanels (approximately 1.5mils 13.75micronsl) of blast-cleaned steel afier
1,200-hour combined corrosion/weathering test

Accelerated Testing of Maintenance Coatings
Charles H. Simpson
is a research chemist with the
Corrosion Science Group at the
Sherwin- Williams Consumer
Division Technical Center in
Cleveland, OH.
Since joining the companyin
1984, he has been involved in
various areas of paint research
and deuelopment.
His current interests include the
investigation ofnew technologies
for the evaluation of
corrosion-resistant coatings.
He obtained a B.S. degree
in Chemistry from Cleveland
State University in 1988.
Brian S. Skerry
is a senior scientist and
project leader of the
Corrosion Science Group.
He has worked as a corrosion
scientist and engineer for the
last 11 years in the UK,
Australia, and the US.
He holds a BB.c. honors degree
in Chemistry (1976) and
M.Sc. (1977) and Ph.D. (1980)
degrees in Corrosion Science
and Engineering from the
University of Munchester, UI{.
Simpson and Skerry
can be reached at the
Sherwin- Williams Company,
601 Canal Road,
Cleveland, OH 44113.
Charles S. Ray
is the manager of the High
Performance Products Group at
the Sherwin- Williams Industrial
Maintenance Coatings
Laboratory in Morrow, GA.
He has 22 years' experience in
product development,
eualuation, and research.
He obtained a B.A. degree in
Chemistry in 1968 from the
Uniuersitu oflllinois at Chicaao
Circle anaa Ph.D. in chemisiry
from Illinois Institute of
Technology in 1978.
Ray can be reached at the
Sherwin- Williams Company,
6795 South Main Street,
Morrow, GA 30260.
tors. This type of testing approach would thus appear
to offer unique advantages over other testing
procedures. An important requirement of any useful
laboratory accelerated test, however, is that it
be able to differentiate the performance capabilities
of closely related coatings of the same generic type.
A test lacking such sensitivity would be of limited
value in product development studies, where it is
often necessary to know how small changes in
coating composition will affect performance.
To address this issue, a study was conducted
in which the cyclic corrosion/weathering test
(as described above) was used to evaluate a series
of experimental acrylic latex paints applied
directly over blast-cleaned steel substrates. These
coatings (designated as coating A, B, C, D, and E)
differ only slightly in their compositional details.
Coated panels were prepared, at equivalent dry film
thicknesses of 1.5 (k 0.25 mils [38 + 6 microns])
using a #75 wirewound drawdown rod. As before, a
scribe was cut into the lower half of each equivalently
cured panel.
Table 5 lists the ASTM blister, rust-through,
and undercutting ratings for the 5 experimental
coatings after a 1,200-hour combined
corrosionlweathering test. These single-coat samples
all exhibited, in varying degrees, formation of
rust spots in areas remote from the scribe as well
as the formation of rust-filled blisters immediately
adjacent to the scribes, as characteristically observed
in the combined corrosion/weathering test.
This is illustrated for 3 of these coatings in Fig. 6,
which demonstrates the effects of incorporating a
corrosion inhibitor (Fig. 6(b)) and a different surfactant
(Fig. 6(c)) on an initial formulation (Fig.
6(a)). Differentiation of the performance capabilities
of these coatings was possible using the combined
corrosion/weathering test. Examination of
failure modes occurring on generically similar
coatings after 12 months' exposure at the marine
site further demonstrated the general "realism" of
these results, because these exhibited a similar
type of scribe line blistering and rust-through in
areas remote from the scribe.
Summary
This article demonstrates the importance of
incorporating weathering factors in accelerated
laboratory tests used to assess the corrosioncontrolling
properties of an organic coating.
The cyclic wetldry corrosion test, with ultraviolet
light condensation weathering factors incorporated,
showed significant promise in meeting the
requirements of a meaningful accelerated test.
Through this testing approach, an improved
reproduction of rankings and failure modes observed
in practice was possible. The wetldry corrosion
test and particularly the continuous salt
spray test were not found to be satisfactory
in this respect. Furthermore, the corrosion1
weathering testing technique described appeared
to have the sensitivity required for applications in
product development programs. Based on the results
obtained in this work, further investigation
using this combined corrrosionJweathering
method seems to be warranted. O
References
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Corrosion of Steel and their Implications," JOCCA,
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2. W. Funke, "Corrosion Tests for Organic Coatings-A Review
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1979, p. 63.
3. B.R. Appleman and Campbell, "Salt Spray Testing for
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Coatings in Salt Spray," Journal >~oafin~s Technology,
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